A Shiite fighter loyal to Iraqi cleric Moqtada al-Sadr holds a position in support of the Iraqi army during a fight against Islamic State militants in the Jurf al-Sakher district on Aug. 18 outside Baghdad. Ali Al-Saadi, AFP/Getty Images

An Iraqi Yazidi woman sits with a child Aug. 17 under a bridge where displaced people of this religious minority found refuge after Islamic State militants attacked the town of Sinjar. Ahmad al-Rubaye, AFP/Getty Images

Supporters of Prime Minister Nouri al-Maliki rally in Baghdad. Tanks and Humvees were positioned on Baghdad bridges and at major intersections with security personnel more visible than usual as pro-Maliki demonstrators pledge their allegiance to him. The placard reads, "al-Maliki is our choice." Karim Kadim, AP

Iraqis chant pro-government slogans in support of embattled Prime Minister Nouri al-Maliki in Baghdad on Aug. 11. Al-Maliki is in a struggle to keep his job after Iraqi President Fouad Massoum nominated deputy speaker of the parliament Haider al-Abadi to replace him. Hadi Mizban, AP

Iraqi army armored vehicles patrol a street in Baghdad, amid tighter security after Iraq's prime minister said he would sue the president in a desperate bid to cling to his job. Sabah Arar, AFP/Getty Images

Peshmerga forces hand out water bottles and show the way to displaced Iraqi families from the Yazidi community as they cross the Iraqi-Syrian border at the Fishkhabur crossing, in northern Iraq. Ahmad Al-Rubaye, AFP/Getty Images

A displaced Iraqi boy from the Yazidi community crosses the Iraqi-Syrian border in northern Iraq. Many from the Yazidi, besieged by jihadists on a mountain in northern Iraq have safely escaped to Syria. Ahmad Al-Rubaye, AFP/Getty Images

An Iraqi Yazidi, who fled her home a week ago when Islamic State militants attacked the town of Sinjar, sits with a baby, on Aug. 10, in a building under construction where Yazidis found refuge in the Kurdish city of Dohuk. Ahmad Al-Rubaye, AFP/Getty Images

Iraqis Yazidis, who fled their homes a week ago when Islamic State militants attacked the town of Sinjar, sit on makeshift beds inside a building in the Kurdish city of Dohuk in Iraq's autonomous Kurdistan region where Yazidis found shelter. Ahmad Al-Rubaye, AFP/Getty Images

Displaced Iraqis from the Yazidi community settle at a camp at Derike, Syria. Kurdish authorities at the border believe some 45,000 Yazidis passed the river crossing in the past week and thousands more are still stranded in the mountains. Khalid Mohammed, AP

Female members of the Kurdistan Workers' Party take position on the front line in Makhmur, south of Arbil, the capital of the autonomous Kurdish region of northern Iraq where clashes with Islamic State militants are ongoing on Aug. 9. Safin Hamed, AFP/Getty Images

An image from British broadcaster Sky television shows Kurdish soldiers battling militants from the Islamic State of Iraq and the Levant on June 18 near Jalula, Iraq. Kurdish security forces fought the Sunni militants, using heavy artillery and rockets to attack their positions. Sky via AP

A photograph released on June 17 by Albaraka News allegedly shows a militant fighter from the Islamic State of Iraq and the Levant tying up a captured Iraqi soldier at an undisclosed location near the border between Syria and Iraq on June 12. Albaraka News via epa

A photograph released on June 17 by Albaraka News allegedly shows fighters from the Islamic State of Iraq and the Levant inspecting vehicles belonging to the Iraqi army after they were captured at an undisclosed location near the border between Syria and Iraq. Albaraka News via epa

Men flash victory signs as they leave a recruiting center to join the Iraqi army in Baghdad. Young Shiite men are joining the army to battle a Sunni militant force advancing from the north. Khalid Mohammed, AP

Militants parade down a main road in Mosul. Days after Iraq's second-largest city fell to the militant fighters, some Iraqis are already returning to Mosul, lured back by insurgents offering cheap gas and food, restoring power and water and removing traffic barricades. AP via Twitter

A photograph from a militant website appears to show militants from the al-Qaeda-inspired Islamic State of Iraq and the Levant taking aim at captured Iraqi soldiers wearing civilian clothing after capturing a base in Tikrit. ISIL has posted photos that appear to show its fighters shooting dead dozens of captured Iraqi soldiers in a province north of the capital Baghdad. AP

An Iraqi army armored vehicle destroyed during fighting with militants sits on a street on June 12 in Mosul. Militants belonging to the Islamic State of Iraq and the Levant have captured Tikrit and Mosul after soldiers and security forces abandoned their posts. AP

Teenagers ride on an armored vehicle belonging to the Iraqi army in Tikrit on June 11. Al-Qaida-inspired militants seized effective control Wednesday of Saddam Hussein's hometown of Tikrit, expanding their offensive closer to the Iraqi capital as soldiers and security forces abandoned their posts following clashes with the insurgents. AP

"I completely disagreed with the decision to walk away from Iraq," said former Army Sgt. Kenneth Mancanares, who served in both Ramadi and Baghdad. "Now, to be honest, I'm trying to think if there's even a way I could get back out there. I'm sure there are a lot of guys feeling that way. I really wish that I could sign up on something tomorrow and join a volunteer group that's going there to stand up for these people."

Mancanares spent more than two years in Iraq, first in Ramadi and then in Baghdad.

"I lost countless friends; I've seen quite a bit of bad stuff over there," he said. "It's not about my personal feelings, about, 'Did I waste my time,' or 'did my buddies die in vain.' I have met those people over there and I became friends with them and I recognize their humanity. That's one of the things that is lost here ... it's so political, no one is really saying how bad it is for those people."

The Sunni Islamic extremist group that has taken Mosul and other parts of Iraq is called Islamic State of Iraq and the Levant, the successor to the al-Qaida group that battled U.S. troops, said Daveed Gartenstein-Ross, a fellow at the Foundation of Defense for Democracies. While ISIL shares al-Qaida's ideology and brutality, it was formally expelled from al-Qaida in February over disagreements on tactics and leadership.

"In terms of core ideological goals, there is a lot of similarity rather than difference, but in the world of jihadism, similar goals don't necessarily mean that organizations will work harmoniously," he said.

ISIL controls a significant part of both Iraq and Syria. In the past week, it has captured Iraq's largest oil field and has come within striking distance of Baghdad. Iraqi troops and police greatly outnumber ISIL fighters, but reportedly have abandoned their posts and fled in the face of in the insurgents' advance, leaving their U.S.-made equipment behind.

Army Sgt. Maj. Rob Bowman served in Mosul and died of cancer in early 2013 after returning home. His wife, Coleen Bowman, said he would be among those angry and frustrated about the new insurgency in Iraq.(Photo: Military Times)

Steven Jerome, a former Army sergeant who served in Tikrit, is especially disappointed with events in Iraq because he remembers the euphoria Iraqis felt during the 2005 referendum on the country's constitution.

"We see those kinds of events here in the U.S. all the time, but these were a first for the Iraqis," Jerome said in an email. "It was exciting to see them take to it with zeal, and satisfying knowing we helped give them that chance. Today, it seems like they lost that drive and just gave up rather than fighting for it. Maybe that was our fault by essentially handing it to them rather than them having to take it."

Former Marine Maj. Lamar Breshears felt the Iraqis were appreciative for being granted freedom from Saddam Hussein, but they were not willing to make the sacrifices necessary to live free on their own.

"This is something that ... we all kind of thought would happen eventually," Breshears said. "So it doesn't come as a surprise. It is painful to watch knowing how much effort and time and money we put into that — lives, blood sweat and tears."

During his deployment to Iraq, former Army Spc. Alex Horton's unit cleared Baquba of al-Qaida in Iraq, which had taken over the city and declared it their capital. The Iraqis were supposed to pick up the ball afterward. Now, he feels all the time and money spent training and equipping the Iraqis was a waste.

Almost all U.S. troops had left Iraq by the end of December 2011 because the Iraqi government failed to reach an agreement that would have allowed a residual U.S. force to remain. While Horton does not believe U.S. troops would have made much of a difference in the security situation if they had stayed in Iraq, he believes U.S. and Iraqi politicians wasted the opportunity for success that U.S. troops created.

"The talks falling apart about keeping a contingency force in there — that had nothing to do with us," he said. "That was a political decision and a political option between Baghdad and Washington. That had nothing to do with an infantry soldier on the ground. We did our end of the bargain and Washington and Baghdad failed to uphold their end."

Amy Staff Sgt. Richard Baca II, who served in Baghdad and Al Nashariya, said he feels the current situation in Iraq is happening because U.S. troops were not allowed to destroy the enemy.

"We would get reports of large amounts of insurgent activity or IED activity or we would tell higher up that the Iraqi police on the ground were telling us that al-Qaida or terrorists were in certain parts of the city and we would want to act upon that and try to root them out — and we would be told that wasn't our mission, our mission was something else," said Baca, who serves with the New Mexico National Guard.

Coleen Bowman's husband, Army Staff Sgt. Rob Bowman, served in Mosul in 2004 and 2005. Despite the dramatic collapse of the Iraqi army, none of the troops who served in Mosul want to hear talk that their service and sacrifice was for nothing, she said.

"They had many, many accomplishments while they were there," Coleen Bowman said. "They left it a lot better than they found it. What happened after that was not up to them. They did their part."

While her husband was in Mosul, he breathed in huge amounts of toxins from dust and smoke that most likely gave him cancer, from which he died after returning home, she said. Still, she is confident that her husband's service in Iraq was worthwhile.

"About four months after he was diagnosed, they did genetic testing on him; they wanted to identify the gene that caused the cancer," she said. "They came back kind of scratching their heads and saying, 'Your body doesn't even have this gene; it's environmental.' "

"I was angry for a couple of months, and he sat down with tears in his eyes and said, 'You've got to stop being angry ... if I could go back and change anything, I wouldn't. I love serving my country and I love leading my men, and I have no regrets.' So, I thought to myself and said, 'If he's not angry and has no regrets, I can't be angry.' "